What is and where do I find my EIP3

The 3rd EIP --also called a "stimulus check" was sent out beginning in mid-March last year.  That was the payment the government gave you of $1400 per person on your tax return.   Do you see any deposits in your bank account for $1400 per person last year?   If you are not sure then check your IRS account.

https://www.irs.gov/payments/your-online-account

The IRS issued letter 6475 to people who received that payment so you should also have a letter.   Enter this information in Federal Review for the recovery rebate credit.   if you did not receive the 3rd stimulus check you can get it as part of your 2021 refund.  It will show up on line 30 of your Form 1040.